Refactor PointerFinder into a separate module

This also parameterize the "excluded pointee types" and exposes a
general method for inserting checks on pointers.

This is a preparation for adding a NullCheck that makes use of the same
code.

compiler/rustc_mir_transform/src/lib.rs

@@ -44,6 +44,7 @@ use std::sync::LazyLock;
 
 use pass_manager::{self as pm, Lint, MirLint, MirPass, WithMinOptLevel};
 
+mod check_pointers;
 mod cost_checker;
 mod cross_crate_inline;
 mod deduce_param_attrs;